A resolving power required in an image scanner is regarded as being different in accordance with an original, for example, 400 dpi (dots per inch) for reading an A3-size sketch or the like, 600 dpi for reading an A4-size document having a high frequency of use as a simple print original, and 1200 dpi for reading a transparent original up to 4.times.5 inches.
In the conventional method responding to such requirements, a plurality of kinds of lenses having different magnifying factors are selectively used in order to change a line density in an image scanner. A general structure of a lens change-over structure in such an image scanner includes, for example, a structure in which each lens is mounted on a slider capable of moving on a rail so that it is selectively positioned in front of an image receiving element such as a CCD (or charge-coupled device) by motor-driving the slider or a structure in which each lens is held on a rotatable disk-like holder so that it is selectively positioned in front of an image receiving element by motor-driving the holder. In such a structure, the positioning of the lens for the image receiving element is made using a limit switch or a proximity sensor.
However, the lens as mentioned above requires a high precision for its optical position relationship. Therefore, for example, in the case of the type in which the slider holding each lens thereon is linearly moved on the rail, a certain degree of precision for the installation of the rail is required over the whole of a moving range of the slider and hence the rail assembling work is liable to become troublesome. Also, in the image scanner as mentioned above, it is general that a scanning unit scans along a horizontal original table mounted with an original sheet and an image received by a mirror is reflected parallel to the original table and is then received by the image receiving element. Accordingly, it is desired in view of operability that the size of the image scanner in a direction of height thereof or a direction perpendicular to the original table should be made as small as possible. However, in the case of the type in which the disk-like holder holding each lens thereon is rotated, the disk-like holder is rotated in a plane perpendicular to an optical path and hence the holder projects in the above-mentioned height direction, which provides a problem that the whole of the image scanner becomes large in size and the operability thereof is deteriorated.
Further, a positional relationship between the actual lens and the image receiving element differs for every lens due to problems inclusive of the precision of assembly. Therefore, in order to accurately make the optical positioning of the lens and the image receiving element by use of the limit switch or the proximity sensor as mentioned above, the position of each sensor itself must be adjusted for every image scanner. This work is liable to be troublesome.
A principal object of the present invention made in light of the above-mentioned problems of the prior art is to provide an image scanner in which the high-precision positioning of a plurality of lenses upon change-over is possible with a simple structure and the size of the whole of a device is not increased.
Another principal object of the present invention is to provide a lens position control structure and control method in an image scanner in which the kinds and/or positions of a plurality of lenses can be accurately detected and controlled with a simple structure.